Laser beam source

ABSTRACT

In a laser beam source for projecting laser pulses, having a laser-active medium ( 10 ), a controllable pump device ( 11 ) for exciting the medium ( 10 ), a resonator ( 13 ) for inducing oscillation of the medium ( 10 ), which resonator has an end mirror ( 14 ) and an output mirror ( 15 ) with an orientation axially parallel to the medium ( 10 ), for the sake of projecting laser pulses of constant pulse energy immediately upon release of the laser beam source, one of the resonator mirrors ( 14 ) is pivotably supported and is pivoted out of its resonator position and back into its resonator position by an actuator ( 19 ) at selectable times (FIG.  1 ).

BACKGROUND OF THE INVENTION

The invention is based on a laser beam source for projecting laserpulses, in particular for machining material.

Such laser beam sources, embodied for instance as flash lamp- ordiode-pumped solid-state lasers, are used among other purposes inmachining material, for instance for welding, drilling or cutting (H.Hügel, “Strahlwerkzeug Laser”, Teubner-Verlag, Stuttgart, 1992). In it,a controllable shutter following the laser beam source and used forprotection purposes is synchronized with the control of the pump devicein such a way that first the shutter opens, and then thepulse-controlled flash lamps or diodes excite (pump) the laser-activemedium, which in the case of the solid-state laser is the laser crystal.This sequence is maintained in order to prevent the closed shutter frombeing burned by the laser pulses, but as a consequence of it, in thefirst laser pulses projected after the activation of the pump devicethere are fluctuations in the pulse energy, which are caused by thethermal transient-response process of the laser-active medium. Thesefluctuations in energy have a very adverse effect on the machiningprocess, especially in single-pulse machining, for instance in weldingor drilling.

SUMMARY OF THE INVENTION

The laser beam source of the invention for projecting laser pulses hasthe advantage that the laser-active medium, such as the laser-activecrystal, is permanently excited and is thus thermally in equilibrium, bypivoting outward of the resonator mirror from its resonator position,but the laser oscillation is interrupted and thus no laser pulse isgenerated. As soon as laser pulses are to be projected, then by pivotingthe resonator mirror back into its resonator position, in which thenormal to the mirror surface is aligned with the normal to the surfaceof the other resonator mirror or is oriented parallel to it, the lasingprocess of the medium is enabled. All the laser pulses projectedthus—since even when the first laser pulses are projected, the thermaltransient-response process has already been concluded—have the samepulse energy, and it is assured that as a result, the outcome ofmachining, from the very outset, meets the quality desired.

With the shutter disposed between the laser-active medium and one of theresonator mirrors, the process of laser oscillation between theresonator mirrors is blocked by the closure of the shutter and set intomotion by the opening of the shutter.

For interrupting the lasing process, only small pivot angles of thepivotably supported resonator mirror are needed, so that in anadvantageous embodiment of the invention, a piezoelectric actuator canbe used as the actuator. The actuator and the pivotable resonator mirrorare advantageously coupled in such a way that when the piezoelectricactuator is not excited, the lasing process is interrupted, or in otherwords the resonator mirror is pivoted out of its resonator position, andthat whenever laser pulses are required, the piezoelectric actuatorexcited via the control unit pivots the resonator mirror back into itsresonator position.

The laser beam source of the invention for projecting laser pulseshaving the characteristics of claim 7 has the advantage that the laserbeam source is always in full operation, and only the beam direction ofthe laser pulses is shifted, so that in pauses during machining thelaser pulses do not reach the machining site.

In one advantageous embodiment of the invention, the deflector device isembodied as a deflector mirror that can be transferred by an actuatorinto two pivoted positions; the orientation of the deflector mirror inits pivoted positions is accomplished such that the beam direction ofthe laser pulses in one pivoted position is aimed at a machining siteand in the other pivoted position is aimed at a so-called beam sump. Theenergy of the laser pulses is dissipated in the beam sump. Once again,the requisite pivot angles of the deflector mirror are relatively small,so that a piezoelectric actuator can be used as the actuator.

In an alternative embodiment of the invention, the deflector device isembodied as an acousto-optical modulator, which by applying ahigh-frequency acoustic power to a so-called Bragg cell diffracts thelaser pulse passing through the Bragg cell to the first order and thuspivots it away from the machining site.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail in the ensuing descriptionin terms of exemplary embodiments shown in the drawing. Shown are:

FIGS. 1–4, a block circuit diagram of a laser beam source, in fourdifferent exemplary embodiments, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The laser beam source shown in FIG. 1 for projecting laser pulses, whichis used particularly for machining material, has a laser-active medium10 in a known manner, which by being supplied with energy is transferredfrom a state of thermodynamic equilibrium into a laser-active state. Theenergy delivery is effected by means of a controllable pump device 11,which is clocked by a control unit 12; the number of pulses, pulsefrequency and pulse length of the laser beam source can be varied bymeans of the clocking rate and the length of the control pulses. Inso-called solid-state lasers, for instance the most importantrepresentative of the category in industrial material machining, whichis the Nd:YAG laser, the laser-active medium 10 is achieved with acrystal, which is doped with metal ions or rare earth ions as alaser-active medium. In the Nd:YAG laser, the so-called host crystal isan yttrium-aluminum garnet, in which Y ions in the crystal lattice arereplaced with Nd (neodymium) ions. The pump device 11 can be a flashlamp or a diode, which is controlled in pulsed form and illuminates thesolid-state body or crystal. By means of a resonator 13, which in thesimplest case comprises a totally reflective end mirror 14 and apartially permeable output mirror 15, some of the excitation energy madeavailable by the pump device 11 is out-coupled by the laser effect aselectromagnetic radiation in the form of laser pulses. The task of theresonator 13 is to send the laser light through the laser-active medium10 multiple times, as a result of which an amplification process ensues,and the laser lases; that is, laser oscillation begins. In this state,an equilibrium is established between the energy delivered to thelaser-active medium 10 and the energy output in the form of laserradiation by the output mirror 15. A so-called shutter 16 is alsolocated behind the output mirror 15 in terms of the beam direction; ithas a protective function and is closed when the laser beam source isturned off. Upon actuation of the laser beam source, the shutter 16 isopened by the control unit 12. A diaphragm 21 with an aperture 22disposed between the laser-active medium 10 and the output mirror 15serves to improve the beam quality of the laser radiation.

To prevent the shutter 16 from being burned by laser pulses, the controlof the shutter 16 is harmonized with the control of the pump device 11in such a way that first the shutter 16 opens, and then the pump device11 pumps the laser-active medium 10. To prevent the projection of laserpulses with fluctuations in pulse energy during the thermaltransient-response process, which ensues at the onset of pumping, in thelaser-active medium 10, which is not yet in thermal equilibrium,blocking means for suppressing the lasing process of the laser-activemedium 10 are disposed in the resonator 13, and they can be deactivatedat selectable times for a selectable length of time. The times fordeactivation of the blocking means are set such that the thermaltransient-response process of the laser-active medium 10 is concluded,and this medium has reached its thermal equilibrium. The selected lengthof time depends on the duration of machining. The deactivationadvantageously occurs between two lighting pulses by the flash lamp ordiode.

In the exemplary embodiment of FIG. 1, the blocking means have a pivotbearing 17 for the end mirror 14 in the resonator 13 and an actuator 18,which adjusts the pivotably supported end mirror 14 and which isembodied here as a piezoelectric actuator 19 controlled by the controlunit 12. As long as the lasing process of the laser-active medium 10 isto remain suppressed despite pumping by the pump device 11, or in otherwords as long as the blocking means are operative, the end mirror 14assumes its position shown in dashed lines in FIG. 1, in which it ispivoted out of its resonator position. The resonator position is definedin that the normals of the two resonator mirrors 14, 15 are aligned withone another or oriented parallel to one another, so that a wave train ofthe electromagnetic waves emitted spontaneously in the laser-activemedium 10 passes through the medium 10 multiple times by reflection fromthe two resonator mirrors 14, 15 and is amplified because of inducedemission until such time as so-called laser oscillation ensues. If theend mirror 14 is transferred to its position shown in dashed lines inFIG. 1, then the wave train reaching it, emitted in the axial directionby the laser-active medium 10, is reflected in a different direction anddoes not arrive back at the medium 10. The amplification process cannotensue in that case, and the laser cannot lase.

If a laser pulse train is required for machining material, then thecontrol unit 12 triggers the piezoelectric actuator 19, and thepiezoelectric actuator pivots the end mirror 14 into the resonatorposition shown in solid lines in FIG. 1, in which the amplificationprocess described above ensues, and the laser beam source projects thepulse train. The control of the piezoelectric actuator 19 is effectedbetween two successive lighting pulses of the flash lamp or diode, sothat the return process of the end mirror 14 is concluded once thesecond lighting pulse has been projected. Since only the lasing processof the medium has been blocked, but not the pumping of the medium 10 bythe pump device 11, the medium 10 is in thermal equilibrium when the endmirror 14 pivots into its resonator position, and all the laser pulsesprojected by the laser beam source have the same, constant pulse energy.

It is understood that is possible, instead of the end mirror 14, also toprovide the output mirror 15 with a pivot and to couple it with thepiezoelectric actuator 19. The mode of operation is the same as thatdescribed above.

In the exemplary embodiment of FIG. 2, the blocking means forsuppressing the lasing process of the laser-active medium 10 have ashutter 20 and an actuator 18 that is actuated by the shutter 20. Theactuator 18 is in turn embodied as a piezoelectric actuator 19, which istriggered by the control unit 12 for opening and closing of the shutter20. The shutter 20 disposed between the output mirror 15 and thelaser-active medium 10 likewise, in the closed state, prevents thelasing of the medium 10, since because of the shielding of the outputmirror 15 from the medium 10, the above-described amplification andoscillation process cannot ensue. Not until the shutter 20 is opened arethe lasing conditions for the laser established, and the laser beamsource projects the laser pulses, whereupon even the first laser pulsehas the same pulse energy as those that follow it.

The shutter 20 can be embodied in various ways. In the exemplaryembodiment of FIG. 2, the shutter 20 has a diaphragm 21 and a shieldingplate 23 that covers the aperture 22 and that is moved by thepiezoelectric actuator 19 transversely to the resonator axis and canthus be moved out of the resonator 13 or into the resonator 13. Themotion can be executed by transverse displacement or pivoting of theshielding plate 23. When a laser pulse train is called up, the controlunit 12 first opens the shutter 16 and then subsequently moves theshielding plate 23 out of the resonator 13, so that the lasingconditions in the resonator 13 are established.

It is understood that it is also possible for the shutter 20 or theshielding plate 23 to be disposed between the laser-active medium 10 andthe end mirror 14 and to be actuated in the same way by thepiezoelectric actuator 19 controlled by the control unit 12. In theexemplary embodiments of the laser beam source in FIGS. 3 and 4, forgenerating laser pulses of constant, unfluctuating pulse energy, anintervention is made not into the resonator 13; instead, a deflectordevice 24 is disposed in the laser beam path from the laser beam sourceto a workpiece 24 that is to be machined, and upon its activation thedeflector device shifts the beam direction of the laser pulses such thatthey do not reach the workpiece 24 but instead reach a so-called beamsump 26, where the pulse energy is dissipated. For calling up laserpulses to the workpiece 24, the activation of the deflector device 25 isdiscontinued again, so that the laser pulses strike the machining site28 on the workpiece 24. Since the laser is already in operation by thetime the first laser pulse is called up to the machining site 28, allthe laser pulses have the pulse energy from the very outset. The layoutof the resonator 13, with the flash lamp 11, the control unit 12, andthe diaphragm 21 located in front of the output mirror 15 and theshutter 16 located behind it is unchanged, so that identical componentsare identified by the same reference numerals.

In the exemplary embodiment of FIG. 3, the deflector device 25 isembodied as a deflector mirror 27, which can be transferred by anactuator 18 into two pivoted positions. The orientation of the deflectormirror 27 in its two pivoted positions is then done in such a way, inagreement with what has been said above, that the beam direction of thelaser pulses in one pivoted position is aimed at the machining site 28on the workpiece 24 and in the other pivoted position is aimed at thebeam sump 26. Once again, the actuator 18 is preferably embodied as apiezoelectric actuator 19, which needs to adjust the deflector mirror 27by only a small pivot angle. It is understood that it is possible toorient the deflector mirror 27 such that when the piezoelectric actuator19 is unexcited, it assumes the position shown in dashed lines in FIG.3, and is converted into the pivoted position shown in solid lines inFIG. 3 by the piezoelectric actuator 19 that is acted upon by a controlsignal from the control unit 12.

In the exemplary embodiment of FIG. 4, the deflector device 25 isembodied as an acousto-optical modulator 29. One such acousto-opticalmodulator is described in terms of its layout and mode of operation inthe catalog “Akusto-Optik” [Acousto-Optics], 1999, for instance, put outby ELS Elektronik Lasersystem GmbH, Groβ-Zimmern. Such anacousto-optical modulator comprises a Bragg cell, in which theinteraction between sound and light takes place. The acoustic power isfed into the Bragg cell by means of a transducer. If the Bragg cell istriggered with a suitable high-frequency power, then periodic changes inthe index of refraction develop in the interior of the cell. The laserbeam or laser pulse passing through is diffracted by this change in theindex of refraction and is directed away from the machining site 28toward the beam sump 26. Otherwise, the layout of the laser beam sourceis equivalent to that described for FIG. 3, and so identical componentsare identified by the same reference numerals.

1. A laser beam source for projecting laser pulses for machiningmaterial, comprising a laser-active medium; a pump device for excitingsaid medium; a resonator for inducing oscillations in said medium, saidresonator having an end mirror and an output mirror each forming a partof said resonator for inducing oscillations in said medium and each withan orientation axially parallel to said medium; a control unit forcontrolling a number of pulses, a pulse frequency, and a pulse length,one of said mirrors of said resonator being supported pivotally; anactuator engaging said one mirror which is pivotally supported, forpivoting said one mirror, which forms a part of said resonator forinducing oscillations in said medium, out of a resonator position andfor pivoting said one mirror back into the resonator position atselectable times so as to influence laser oscillations.
 2. A laser beamsource for projecting laser pulses for machining material, comprising alaser-active medium; a pump device for exciting said medium; a resonatorfor inducing oscillations in said medium, said resonator having an endmirror and an output mirror each with an orientation axially parallel tosaid medium; a control unit for controlling a number of pulses, a pulsefrequency, and a pulse length, one of said mirrors of said resonatorbeing supported pivotally; an actuator engaging said one mirror which ispivotally supported, for pivoting said one mirror out of a resonatorposition and for pivoting said one mirror back into the resonatorposition at selectable times, wherein said control unit is farmed so asto control said actuator so that the pivoting back of said one mirror ofsaid resonator and opening of a shutter are each effected between twoexcitation pulses of said pump device.
 3. A laser beam source as definedin claim 1; and further comprising a shutter which intervenes betweensaid laser-active medium and said one mirror of said resonator.
 4. Alaser beam source for projecting laser pulses for machining material,comprising a laser-active medium; a pump device for exciting saidmedium; a resonator for inducing oscillations in said medium, saidresonator having an end mirror and an output mirror each with anorientation axially parallel to said medium; a control unit forcontrolling a number of pulses, a pulse frequency, and a pulse length,one of said mirrors of said resonator being supported pivotally; anactuator engaging said one mirror which is pivotally supported, forpivoting said one mirror out of a resonator position and for pivotingsaid one mirror back into the resonator position at selectable times,wherein said actuator has a piezo-electric actuator.
 5. A laser beamsource as defined in claim 2, wherein said output mirror is followed, ina laser projection direction, by a protective shutter, said protectiveshutter being triggered by said control unit so that it opens beforepivoting the one mirror of the resonator back again and before openingsaid shutter.